1. Field of the Invention
The present invention relates to the welding of thermoplastic sheets and, more particularly, to processes, machines and products involving welding a plurality of thermoplastic strata together for use in various products, for example, heavy-duty plastic bags, liners and covers. Difficulties have been encountered in producing seams of uniform appearance and strength in certain thermoplastic materials, particularly polybutylene, which melts at 226.degree. F. and tends to remain tacky even after being cooled below that temperature. Specifically, it has been difficult to produce strong welds in polybutylene sheets because of their characteristic polymeric structure, particularly when of heavy gauge, and it has been difficult to handle sheets, particularly in existing equipment, in which welds have been produced because of the tendency of such welds to remain tacky even when solidified.
2. Summary of the Invention
The primary object of the present invention is the provision of processes, machines and products involving the production of a linearly extending straight or curved seal or weld between two or more susceptible thermoplastic sheets, particularly polybutylene sheets, in the following manner. A relaxed skirt at the edges of the superposed sheets is established by clamping edgewise portions of the sheets at relatively high pressure between a pair of heat conducting members. The skirt is fused by heat at relatively low pressure for a predetermined time to form a weld having a generally round cross section at the edges of the sheets. The weld then is subjected to an aqueous surfactant fluid to control its tackiness so that the product can be handled immediately. Preferably, the heat is applied to the skirt by an elongated manifold which establishes a line of low pressure gas flame that parallels the skirt at the edge of the clamp. Preferably, the seal is substantially transverse with respect to the axial orientation, particularly the monoaxial orientation of both of the thermoplastic sheets. The resulting weld, in cross section, is generally round throughout most of its periphery and joins the sheets at fillets. In various forms, the aqueous surfactant is a mist in which aqueous detergent droplets are propelled by a stream of air, i.e. a mist having an inner phase composed of aqueous detergent and an outer phase composed of air, a liquid stream which flows over the weld, or a liquid bath through which the weld is advanced.
The process of the present invention is applicable to the sealing of thermoplastic sheets and is needed, particularly for the sealing of polybutylene sheets to produce polybutylene products having panels. The present invention speeds formation of a stable seal in order to minimize production delays, which, in the past, have resulted from long solidification and recrystallization times.
Other objects of the present invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the processes, machines, and products, together with their steps, parts, components and interrelationships, the scope of the disclosure of which will be indicated in the appended claims.